Dual aerating flotation machine



Sept. 8, 1953 Filed June 14, 1948 A. C. DAMAN DUAL AERATING FLOTATION MACHINE 4 Sheets-Sheet 1 INVENTOR. Arth u r 0. Damon ATTORNEY FIG-6 Sept. 8, 1953 A. c. DAMAN 2,651,413

DUAL AERATING FLOTATION MACHINE Filed June 14, 1948 4 Sheets-Sheet 2 INVENTOR. Arthur 0. Damon ATTORNEY c. DAMAN DUAL AERATING FLOTATION MACHINE Sept. 8, 1953 4 Sheets-Sheet 3 Filed June 14, 1948 INVENTOR. Arthur 0. Damon BY ATTORNEY Sept. 8, 1953 A. c. DAMAN DUAL. AERATING FLOTATION MACHINE 4 Sheets-Sheet 4 Filed June 14, 1948 INVENTOR. Arthur 0. Damon diam A TTORNEY Patented Sept. 8, 1953 UNITED STATES PATENT OFFICE DUAL AERATINGT FLOTATION MACHITNE Arthur C. Daman, Denver, (3010;, assignor to Mining Process and Patent Company, Denver, 0010., a, corporation of Delaware;

Application June 14, 1948, Serial No. 32,936 13 Claims. (01. 209- -168) This invention relates to frothflotation treatments and apparatus, and more particularly relates to flotation apparatus designed to provide a given amount of aeration to the entire body of pulp under treatment and another amount of aeration to the non-floated'solids portion of the pulp.

In the past, it has been customary in froth flotation practice to aerate a conditioned pulp, or to mix gas and reagent with pulp in an agitation zone so as to induce flotation of selectively coated particles, while other pulp constituents that do not float and carry intothefroth are passed through the succession of treatment stages and finally are discharged from the treatment.

In some instances the former practice'was not satisfactory, due to the fact that aconsiderable portion of the pulp constituent which should carry into the froth was imperfectly coated or otherwise failed to elevate sufficiently to reach the froth bed, and remained in suspension in a zone immediately below said froth bed, increasing in density through continuance of the action, until the density of material in saidzone impeded the flotation of the properly coated mineral.

Various methods have been devised to-counteract these conditions; Unfortunately a mechanical type flotation machine has to perform three functions, namely: pumping; aerating; and recirculating pulp, particularly the high density portion generally designated middlings. The impeller. is utilized inperforming all such functions, and when itis operated primarily as a pump, it reduces in efficiency as an aerator and recirculating means. Similarly, if it aerates .satisfactorily it is not efficient as'a pumper and recirculator.

The present invention provides a. froth flotation treatment in which the mechanical aerator provides the desired degree. of pumping,.aeration and recirculation to satisfy theirequirements of any given treatment. In addition, this invention provides aselective control of thevolume or point of introduction of the aeratinggas, while additional provision is made for the movement of settled ornon-floated material directly through the succession of treatment stages independently of such aerator action. This eliminates or substantially reduces the need to utilize the aerator as a pumper, and in its preferred form .provides enough aeration so that the aerating function of theaerator may bev reduced a substantial amount without impairment of the aerating efiiciencyof the treatment.

Another novel feature of the present invention is incorporated in multi-cell treatments. Provision is made for the passage of non-floated solids, usually granular material, directly through the succession-of-treatment stages without intermingling with-the floated or floatable constituents, and particularly the slimes constituents, of the treatment. While this action is illustrated in connection with a preferred form of apparatus, it is capable of utilization in various types of multi-cell, mechanical-type flotation machines.

Thus, it is apparent that the present invention fills a long-felt want. It is an object of the present invention to provide simple, efiicient and economical methods of performing the froth flotation treatment.

Another object of the present invention is to provide simple, durable and" efficient froth flotation apparatus in which'controls are provided to furnish a desired degree of pumping, aeration and pulp-recirculation.

A further object of the present invention is to providea selective control of the velocity and place where an aerating gas is introduced into a. froth flotation cell. 7

Other objects reside innovel details of construction and in novel combinations and arrangementsof parts, all of which will be fully described in the courseof the following description.

The practice of the invention will be best understood. by reference to the accompanying drawings- In the drawings, in the several views of which like parts have been designated simi larly,

Fig. 1 is a verticalsection through a multicell flotationmachine embodying structural featuresof the present invention and capable of performing the novel methods of this invention;

Fig. 2 is a developed Tsection takenalong a line corresponding. to the line Z2, Fig. 1;

Fig. 3-is a fragmentary section taken approximately on the line.3-3, Fig 1;

Fig. 4 is an enlargedfragmentary front elevation of a typical unit-cell aerator, partially broken away to show interior parts, and embodying features of the present invention;

Fig. 5..is a fragmentary front elevation illustrating another form. of unit-cell areator employing a different method of introducing gas into the hollow shaft; and

Fig. 6 is a fragmentary vertical section of another impeller unit. utilized in the machine of the present invention.

As shown in Fig. l of the drawings a typical multi-cell machine will comprise a tank T having front and rear walls I and B, a bottom 9, a forward end wall l and a rear wall and tailings discharge box or compartment (not shown). The interior of said tank is divided into a series of cells, here shown as three in number and designated C, C and C respectively.

In some instances it will be desirable to provide a combined overflow and feed-in compart-r ment i2 between adjoining. cells, and in such event the intermediate end wall 13 will be provided with an overflow weir [4, here shown as of the slat type, and a bailie l supported in spaced relation to said weir to restrain froth movement to said weir. The forward walls 16a or 151) of cells C and C cooperate with the forward end wall IE] or an intermediate end wall l3, respectively, to form the feed compartment of the respective cells.

In the machine illustrated in Fig. 1 a novel type of circulation of sands or non-floated granular material is provided. I prefer to locate one or a plurality (here shown as 2) Venturi passages 17 of hourglass shape between adjoining cells, and a conduit l8 controlled by valve 59 discharges gas under relatively high pressure into the narrow portion of the passage so as to induce a forceful movement of the sands or granular material directly from one cell to the next in a progressive movement through the machine. In some instances it will be desirable to provide a sands discharge outlet 28 in addition to the Venturi passage, and I prefer to locate this outlet in wall 13 substantially in alignment with another opening in wall l6b, from which a feed conduit 22 extends for delivery of the pulp to an impeller unit as will be subsequently described. As shown in Fig. 2 a suitable froth overflow is provided for each cell, and I prefer to employ a slat type weir 21! on both front wall I and rear wall 8 for regulating the froth overflow, although if desired a single overflow weir may be utilized.

The aerating features of the present invention will be best understood by reference to Fig. 4 as the impeller arrangement and gas discharge system illustrated therein is typical of the aerating unit used in the several cells shown in Fig. 1. In such an assembly I provide a hollow shaft 23 journalled in a double bearing member 24 supported from superstructure 25 at the top of a given cell by adjustable plate members 260. and 26b. The shaft is driven from. a motor 28 or other prime mover by transmission 29, here shown as a V-belt drive. Two impellers an and 3! are mounted on the shaft; the lower impeller being positioned in relatively close proximity to the bottom 9 of the tank or cell, and is of substantially greater diameter than impeller 3| which preferably is located on the shaft at an elevation below or in proximity to the froth bed of the cell. Both of these impellers may be of any suitable type and as here illustrated are the standard impeller of Denver Equipment Company (Patent 2,031,590). A suitable aerating gas from a supply line 33 is delivered into the hollow shaft 23 and discharged therefrom at different elevations through a series of apertures 34, 35,,-

and 36, as well as through the open bottom end 31 of said shaft. The apertures 34 preferably arranged at 90 intervals about the shaft are located immediately above impeller 31, while the aperture 35 here shown as one but which may be a plurality, is located immediately below im- 4 peller 3|; and aperture 36, here shown as one but which may be a plurality, is located immediately above impeller 36.

As will be explained in the subsequent description, it is desirable to vary the volume of gas discharge, and in some instances the point of discharge through the respective apertures. To this end I provide sleeves 38, preferably of rubber or other wear-resistant flexible material, which are held in frictional engagement with shaft 23 and are movable along the shaft to partially uncover or completely cover said openings as required. This arran ement permits an upper or a lower portion of the opening to be exposed and thus provides variation in the point of discharge position, while the range of adjustment from fully opened to fully closed position provides the volume adjustment of this control.

I prefer to operate each of said impellers within a complete or at least a partial enclosure. As shown in Fig. 4 a tubular enclosure or cover 49 is provided for impeller 30 and is so arranged as to provide a flanged or flaring portion 4| overhanging the impeller, and in close proximity to the top surfaces of the impeller blades. The inner end of the feed conduit 22, previously described, extends into a suitable opening in enclosure 40 so that feed from a feed compartment [2 is delivered directly through the feed conduit 22 into this impeller enclosure. In order to provide a proper control of the rate of feed supply and also to insure gas retention within the enclosure, I provide a flap valve 42 within the conduit 22 and regulate the setting of this valve by a rod 43 connected thereto, which extends above the liquid level in the cell. At its upper end the enclosure 40 tapers inwardly to provide a narrow throat portion 44 enclosing and only slightly spaced from shaft 23. A cover plate 45 is fixed on shaft 23 immediately above throat 44 for conjoint rotation with the shaft, and a rubber sleeve 46 fitted on shaft 23 substantially fills the space between throat 44 and plate 45 to function as a gasket and prevent the escape of any substantial quantity of gas from the errclosure.

Additional aerating gas which may be the same gas as that supplied through shaft 23, or a different gas according to the requirements of the treatment, is delivered from a suitable supply source (not shown) through a valve controlled conduit 41 for discharge into enclosure 40. The enclosure member 40 constitutes a stationary assembly and is suitably supported as by baffles (not shown) extending from the walls of the cell or supported on its bottom to provide a stabilizing support therefor. In addition, the feed conduit 22 through its connection with the walls 16a or 182) also functions as a stabilizing part of the assembly.

The upper impeller 3| is enclosed except on its bottom by a hood member 49 which preferably is dish-shaped as shown and is supported from superstructure 25 by adjustable straps or brackets 50, which permit up and down movement of hood 49 to selectively vary its spacing from the top of the impeller blades. In the preferred arrangement, hood 49 has a central annular passage EI and at least one peripheral aperture 52 positioned at approximately the same elevation as the impeller blades. In order to prevent too rapid centrifugal movement of the pulp mixed within the enclosure formed by hood 49 and its "associated impeller, I prefer to locate a series of vanes 54- on the inner surface of the hood positioned substantially in the plane of the impeller so asto break up and restrain the pulp streams tending to assume a spiraling course along the inner surface of the hood in their passage to the periphery of same.

A baffle 53, preferably in the form of an an.- nulus, extends downwardly from superstructure 25 in overhanging relation to hood 49 and is submerged to a sufiicient depth to prevent surface froth from being drawnthrough opening 5| into the upper impeller enclosure. While suitable aeration for this impeller usually is provided by the discharge through openings 34.and

35, I prefer to locate a bonnet. member 55 beneath bearing member 24 with its lower end'open and in close proximity to the annular aperture 5|. Gas from a suitable source of supply. (not shown) is delivered through a valve controlled branch line 56 into theinterior of bonnet 55., and-being free to escape only through the open bottom, is entrained by the vertex action and carried into the impellerenclosure through opening 5!.

Another type of gas discharge control is utilized with opening 35. In place of the rubber sleeve 38, a plug (not shown) may be inserted in this aperture to completely close same, or if it is desired to discharge a lesser volume of gas, the plug may be provided with a restricted orifice as a gas-escape passage.

InFig. 5 I have shown another method of introducing gas into the hollow shaft 23. Instead of directing the gas into the open upper end of said shaft 23, the double bearing unit 241: of this form is apertured to receive a nipple 58 into which a supply line 33a: is fitted. The upper end of shaft 23 is closed by a cap member 59, and consequently the gas discharged by line 33a: can escape only through the lower outlets in shaft 23.

In certain treatments, it will be desirable to provide additional aeration to that described hereinbefore and to this end a supply line 60 connected with a suitable source of gas supply (not shown) extends through cell bottom 9 and discharges directly beneath impeller 30. This gas mingles with the pulp at the bottom of the cell and is promptly subjected to the centrifugal influence of the impeller. Some of the entrained gas rises in proximity to the enclosure 40. and is drawn through one or a plurality. of recirculating openings 6| in the enclosure for mixing with pulp therein.

With this understanding of the structural features utilized in the apparatus of the present invention, the preferred operating procedure will now be described.

Referring first to Fig. 1, any suitable flotation pulp is introduced into the machinethrough a feed inlet 62 and fills the feed compartment [2 ahead of cell C to a depth sufiicient to submerge feed conduit 22. At the same time a portion of the non-floated material immediately below the froth bed is overflowed through an opening 63 into the feed compartment [2. Thus, the incoming feed to the lower impeller 30 is a partially aerated pulp which receives additional aeration from the gas introduced through enclosure 40, the bottom outlet 31 of shaft 23 and through supply line 60, as well as from other aerated pulp recirculated through opening 6|.

If the feed through conduit 22 of cell C tends to diminish to the point where gas could escape to atmosphere, the flap valve 42 is moved by r d 43 to restrict the flow through saidconduit and thus produce sufficient hydrostatic head in, compartment l2 to prevent escape of the gas to at.- mosphere.

Int he usual flotation treatment a previously conditioned pulp will be subjected to the aeratin influences. However, if it is necessary to feed a flotation. reagent directly to a cell, such as the cell C in Fig. 1, such reagent may be fed through a suitable feeder directly into feed compartment l2, or through hollow shaft 23, or through conduit; 4'! or otherwise onto impellers 30 and 3 I.

In any even pulp, reagent and aerating gas are brought together within the confines of the flotation cell, and subjected to intimate intermixing in a circulatory movement with the result that the gas is distributed through the pulp. body in dispersed condition and rises to the surface, collecting the selectively coated mineral or other valuable constituent in a froth on the surface of the pulp body.

Where, as frequently happens, same of the valuable constituent tends to gather underneath the froth bed and form a zone of relatively high density, which resists the penetration of floating material, it is necessary to provide additional aeration and mixing in order to increase the floatability of such material. The dual aerator of the present invention effectively performs this func-. tion in that the lower impeller unit can be utilized to provide the necessary mixing, recirculation and aeration for normal flotation requirements; while the upper impeller unit functions primarily as a recirculator to provide additional mixing'and aeration to the middlings fraction of the pulp, that otherwise would form the high density zone beneath thefroth bed. For this reason, the hood 49 is located just beneath the froth bed so as to position the annular opening 5| at an elevation where it will effectively form a vortex to entrain the iiddlings under the pumping influence of the upper impeller.

From the foregoing it will be apparent that the impeller units function primarily as mixers andaerators, rather than as circulating pumps. In performing the process of the present invention, Iprefer to utilize an operation in which nonfloatedconstituents of the respective cells are drawn or moved through the succession of cells by the Venturi action induced in the several submerged passages H. A gas under pressure, usually compressed air, is discharged into each such passage and directed toward the successing cell of the series.

Due to the positioning of such passages I! ate location outside the plane of the several impellers of the series, at least one portion of the material so moved through the passage is subjected to an augmenting movement induced by the centrifugal action of the impeller, while the other portion of impeller discharge which is in direct opposition to said Venturi movement induces an arrested action or static condition which insures adequate aeration and ultimate flotation of a maximum portion of the fioatable constituents of the pulp.

The gas discharged into and through the venturi passages functions primarily as a motive force, but as the impelling movement dissipates,

the gas rises to the surface and this contributes to canbe rotated at optimum aerating speeds rather than optimum pumping speeds and still supplement to Venturi action to a degree sufficient to produce efilcient circulation of the pulp.

Thus in the machine illustrated in Fig. 1, pulp passes from cell C to cell through the passage l1 and also over weir [4 while froth overflows the weirs 2| and is discharged from the machine. If desired, other pulp may be circulated in cell C through opening 63, by setting its associated gate at an open position. This pulp mixes with incoming feed in feed compartment 12 and is delivered to impeller 30 through feed conduit 22. Also, if desired, reagent may be fed into compartment l2 and premixed with the pulp prior to its delivery onto impeller 30.

Pulp entering cell C is subjected to a similar treatment, with the exception that the discharge for granular tailings from cell C is through the Venturi passage ll, while other suspended solids or slimes not carried into the froth discharge pass from the cell 0 through an opening 66 in wall [61). This arrangement is particularly desirable when a substantial portion of the valuable constituent of the feed to the first cell is removed as a concentrate from such cell. In such event, the solids content to the second cell C is substantially less than in the first cell and hence it will be desirable to discharge the granular content rapidly and retain the solids or slimes content in cell C for a longer treatment interval. The pulp discharged from cell C into cell C is ready for final treatment as cell C will comprise the final cell of the series.

On this account the latter cell arrangement may be in either the form shown in cell 0 or cell C as determined by the requirements of the treatment. Since these structural arrangements are only duplications of the structure hereinbefore described, cell C has been illustrated in fragmentary form. In each of these cells an aerating arrangement of the general type shown in Fig. 4 will be employed. While I have described a typical three-cell machine, it will be understood that cell C represents any number of intermediate cells in the series. Also, additional cells of the cell C type may be used as intermediate cells in the series. When cell C is the final cell, it will have a discharge contro1 provided by a Wall and weir arrangement of the same type as wall l3, weir M of cell C and a box to receive said discharge corresponding to the compartment l2 between cells C and C For certain operating conditions a different type of aeration may be required and this feature has been illustrated in Fig. 5. The hood or cover element disc for the lower impeller 30a: is provided with underhanging blades 61 and has an opening or socket portion to receive the discharge end of feed conduit 22. A hollow column 409: is supported by and extends upwardly from hood Mar: to a point above the liquid level of the cell, and has a series of recirculation openings 68 for entraining pulp.

This column is fitted in frictional engagement with the lower end of flaring bonnet member 551e, suspended from a dual bearing member 24.1: of the same type as bearing unit 24 previously described. However, as shown, the upper and lower bearings are equipped with a grease and air seal 69 to prevent escape of gas along the shaft 23 and from the bearing enclosure. Gas under pressure is delivered through the inlet 33m into the bearing enclosure, then passes through an opening 64 into the hollow shaft 23a: which is capped at its top by a closure 59, to prevent escape of gas except through its lower portion. To this end, the shaft 233: is provided with a series of intermediate outlets 35:0, 35y and 361: as well as a bottom outlet 31:11. In this form, I prefer to locate stationary baflies 10 on cell bottom 9 to cooperate with blades 61 in restraining the agitation induced by impeller 30m.

A pumping type impeller 3lx is mounted for conjoint rotation with shaft 23x above but in proximity to the outlet 35.12. An opening 65 in bonnet 55:1: may entrain atmospheric air under the pumping influence of impeller 3111:, or, if desired, may be connected with a suitable source of gas supply for introduction of a pressure gas into the column enclosure. The impeller 31:1: of this form acts as a fan or a blower to force the air or other gas entering through opening 65 to the impeller 30:1,, while at the same time a portion of the gas introduced into the hollow shaft from line 33a: escapes through outlets 35m and 36a: to aerate pulp delivered onto the upper surface of the impeller 301:. I employ the V-shaped blade arrangement shown in Fig. 5 so that impeller 31x may be used in either right or left hand rotation. Pulp entrained through openings 68 mixes with the gas being forced down column 409: and is subjected to further mixing by impeller 30x. Other gas passes through the lower shaft opening 31m and serves to aerate pulp adjacent the under surface of the impeller.

If additional aeration is required in the lower portion of the cell, such air may be introduced through conduit 60 or said conduit may be connected with a source of pulp feed, such as a rougher concentrate from another cell. Through this arrangement it is possible to introduce different gases at different points in the treatment zone and also to introduce gases under difierent pressure at different points where such action is desirable.

From the foregoing description of the several structural forms and arrangements illustrated in the drawings, it will be apparent that controlled aeration and controlled circulation is provided in all such units whether they be unit cells or multi-cell assemblies. While I have shown the Venturi passage circulating feature in association with other novel machine features of the present assembly, it will be apparent that such circulating control may be employed in various types of mechanical flotation machines either with or without the aerating control of the present invention. In particular, when it is desirable to move the granular product through a machine faster than the slimes product, the Venturi passage arrangement may be utilized irrespective of the form of the agitation and aeration unit.

Another feature disclosed in the drawings which is highly beneficial to the treatment is the provision of valve 42 in feed conduit 22. With this arrangement it is possible to throttle the feed and utilize the air injectors.

Under certain circumstances it will be desirable to provide a different type of mixing in the upper impeller element. A suitable arrangement for this purpose has been illustrated in Fig. 6 in which the impeller 31m comprises a doublewalled hollow body formed to provide an upper central intake opening 5 Im and a peripheral outlet opening 10. The passage H extending between intake 51m and outlet 10 is arranged to impart a downward and outward component of movement to the pulp entering intake 51m. As depicted, the structure in Fig. 6 would be substituted for the impeller 3i and companion hood 49 shown in .cell C of Fig. l, for example.

Certain of the novel features of the present invention have been shown and described in my copending application Serial No. 754,653 entitled Pumping-Type Flotation Apparatus and Method. The features described but not claimed herein have been claimed in the aforesaid application.

Having thus described the invention, what is claimed is:

1. In a froth flotation machine, in which pulp is circulated through a succession of cellsand subjected toaeration in each cell for the collection of a pulp constituent in a froth on the surface of the body of pulp under treatment, the improvement which comprises an open-ended horizontal passage disposed above and adjoining the bottom of the machine and extending between a preceding cell and a succeeding cell in the succession of cells, said passage including a narrow portion and an adjoining enlarged portion discharging into the succeeding cell, and conductive means for directing gas under pressure into the narrow portion and through the enlarged portion of said passage toward the succeeding cell at suflicient velocity to induce movement of a substantial portion of the pulp solids from the preceding cell to the succeeding cell.

2. In a froth flotation machine, in which pulp is circulated through a succession of cells and subjected to aeration in each cell for the collection of a pulp constituent in a froth on the surface of the body of pulp under treatment, the improvement which comprises an open-ended horizontal passage disposed above and adjoining the bottom of the machine and extending between a preceding cell and a succeeding cell in the succession of cells, said passage including a narrow portion and an adjoining enlarged portion at each end of the narrow portion, and conductive means for directing gas under pressure into the narrow portion and through the enlarged portion of said passage toward the succeeding cell at suflicient velocity to induce movement of a substantial portion of the pulp solids from the preceding cell to the succeeding cell. 3. In a froth flotation machine, in which pulp is circulated through a succession of cells and narrow portion, and conductive means for directing gas under pressure into the narrow portion and through the enlarged portion of said passage toward the succeeding cell atsuflicient velocity to induce movement of a substantial portion of the pulp solids from the preceding'cell to the succeeding cell.

4. In a froth flotation machine, in which pulp is circulaitedthrough a succession of cells and subjected to aeration in each cell for the collection of a pulp constituent in a froth on the surface of the body of'pulp under treatment, the improvement which comprises an open-ended horizontal passage of hourglass shape =-dis posed above and adjoining the bottom of the machine and extending between a preceding cell and a succeedingcell in the succession of cells, said passage including a narrow portion and an adjoining enlarged portion, discharging into the succeeding cell, and conductive means for directing gas under pressure into the narrow portion and through the enlarged portion of said passage toward the succeeding cell at sufficient velocity to induce movement of-asubstantial portion of the pulp solids from the preceding cell to the succeeding cell.

5. In froth flotation apparatus, a flotation cell having a feed inlet and at least one discharge outlet determining a liquidlevel therein, upper and lower rotary impellers in the cell, a cover member overhanging the upper impeller and having a central intake opening below and in proximity to the liquidlevel of the cell and having downwardly bent surfaces extending substantially vertically and laterally below the periphery of the impeller and imparting a downward component to the pulp discharging across the periphery of said impeller, a hood covering the lower impeller connected to theinlet and having control means for the selective recirculation of pulp from the cell, whereby said lower impeller exerts a pumping influence on circulating pulp, and means for separately controlling the air supply to each impeller.

6. In froth flotation apparatus, a flotation cell having afeed inlet and at least one discharge outlet determining a liquid level therein, upper and lower rotary impellers in the cell, a cover member overhanging the upper impeller and having a central intake op'eningbelow and in proximityto the liquid level of the cell and having downwardly bent surfaces extending substantially vertically and laterally below the periphery of the impeller and imparting a downward component to the pulp discharging across the periphery of said impeller, a hood covering the lower impeller connected to the inlet and having control means for the selective recirculation of pulp from the cell, whereby said lower impeller exerts a pumping influence on circulating pulp, means for separately controlling the air supply to each impeller, and means for varying the volume of gaseous discharge to each impeller.

'7. In froth flotation apparatusa flotation cell having a feed inlet and at least onedischarge outlet determining a liquid level therein, upper and lower rotary impellers in the cell, acover member overhanging the upper impeller and having a central intake opening below and in proximity to the liquid level of the cell and havingdowm wardly bent surfaces extending substantially vertically and laterally below the periphery of the impellerand imparting a downward component to the pulp discharging across the periphery of said impeller, a hood covering the lower impeller connected to the inlet and having control means for theselective recirculation of pulp from the cell, whereby said lowerimpeller exerts a pumping influence on circulating pulp, means for introducing separate streams of gas of different composition onto .the upper impeller, and means for separately controllingthe air supply to each impeller.

8. In froth flotation apparatus, a flotation cell having a feed inlet and at least one discharge outlet determining a liquid .level therein, avertical shaft in the cell, upper and lower impellers on said shaft, a cover member overhanging the. upperimpeller and having a central intake opening below and in proximity to the liquid level of the cell and having downwardly bent surfaces adjacent the periphery of the impeller imparting a downward component to the pulp discharging across the periphery of said impeller, a hood covering the lower impeller connected to the inlet and having control means for the selective recirculation of pulp from the cell, whereby said lower impeller exerts a pumping influence on circulating pulp, outlet means for discharging an aerating gas from the shaft onto each said impeller, and a regulating member movable along said shaft for varying the discharge capacity of each said outlet.

9. In froth flotation apparatus, a flotation cell having a feed inlet and at least one discharge outlet determining a liquid level therein, a vertical shaft in the cell, upper and lower impellers on said shaft, a cover member overhanging the upper impeller and having a centralintake opening below and in proximity to the liquid level of the cell and having downwardly bent surfaces extending substantially vertically and laterally below the periphery of the impeller imparting a downward component to the pulp discharging across the periphery of said impeller, a hood covering the lower impeller connected to the inlet and having control means for the selective recirculation of pulp from the cell, whereby said lower impeller exerts a pumping influence on circulating pulp, means for separately controlling the airsupply to each impeller, and regulating means including vertically-adjustable shaftcarrying members positioned above the liquid level in the tank for varying the spacing between each said impeller and its cover.

10. In froth flotation apparatus, a flotation cell having a feed inlet and at least one discharge outlet determining a liquid level therein, upper and lower rotary impellers in the cell, said upper impeller being of substantially less diameter than the lower impeller, a cover member overhanging the upper impeller and having a central intake opening below and in proximity to the liquid level of the cell and having downwardly bent surfaces extending substantially vertically and laterally below the periphery of the impeller imparting a downward component tothe pulp discharging across the periphery of said impeller, a hood covering the lower impeller connected to the inlet and having control means for the selective recirculation of pulp from the cell, whereby said lower impeller exerts a pumping influence on circulating pulp, and means for separately controlling the air supply to each impeller.

11. In froth flotation apparatus, a flotation cell having a feed inlet and at least one discharge outlet determining a liquid level therein, upper and lower rotary impellers in the cell. a cover member overhanging the upper impeller and having a central intake opening below and in proximity to the liquid level of the cell and having downwardly bent surfaces extending substantially vertically and laterally below the periphery of the impeller imparting a downward component to the pulp discharging across the periphery of said impeller, a hood covering the lower impeller connected to the inlet and having control means for the selective recirculation of pulp from the cell, whereby said lower impeller exerts a pumping influence on circulating pulp, means operable from outside the cell for varying the volume of feed delivered. through said inlet to the lower impeller, and means for separately controlling the air supply to each impeller.

12. In froth flotation apparatus, a flotation cell having a feed inlet and at least one discharge outlet determining a liquid level therein, a vertical shaft in the cell, upper and lower impellers on said shaft, a cover member overhanging the upper impeller and having a central intake opening below and in proximity to the liquid level of the cell and having downwardly bent surfaces adjacent the periphery of the impeller imparting a downward component to the pulp discharging across the periphery of said impeller, a hood covering the lower impeller connected to the inlet and having control means for the selective recirculation of pulp from the cell, whereby said lower impeller exerts a pumping influence on circulating pulp, outlet means for discharging an aerating gas from the shaft onto each said impeller, another outlet in said shaft intermediate the upper and lower impellers for discharge of gas into the portion of the pulp body intermediate said impellers, and a regulating member movable along said shaft for varying the discharge capacity of each said outlet.

13. In froth flotation apparatus, a flotation cell having a feed inlet and at least one discharge outlet determining a liquid level therein, a vertical shaft in the cell, upper and lower impellers on said shaft, a cover member overhanging the upper impeller and having a central intake opening below and in proximity to the liquid level of the cell and having downwardly bent surfaces adjacent the periphery of the impeller imparting a downward component to the pulp discharging across the periphery of said impeller, a hood covering the lower impeller connected to the inlet and having control means for the selective recirculation of pulp from the cell, whereby said lower impeller exerts a pumping influence on circulating pulp, outlet means for discharging an aerating gas from the shaft onto each said impeller, another outlet in said shaft intermediate the upper and lower impellers for discharge of gas into the portion of the pulp body intermediate said impellers, means for varying the discharge capacity of the latter outlet, and means for separately controlling the discharge of the other said outlets.

ARTHUR C. DAMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,195,453 Fagergren Aug. 22, 1916 1,240,824 Clawson Sept. 25, 1917 1,324,139 Ziegler Dec. 9, 1919 1,346,286 Waterhouse July 13, 1920 1,374,445 Greenwalt Apr. 12, 1921 1,383,881 Thomas July 5, 1921 1,471,332 Greenwalt Oct. 23, 1923 1,505,324 Ebernez Aug. 19, 1924 1,518,010 Simpson Dec. 2, 1924 1,556,083 Daman Oct. 6, 1925 1,912,228 Shimmin May 30, 1933 2,101,331 Fagergren Dec. 7, 1937 2,104,349 MacCamy Jan. 4, 1938 2,167,788 Weinig Aug. 1, 1939 2,189,779 Daman Feb. 13, 1940 2,232,388 Ingalls Feb. 18, 1941 2,393,976 Damar et al Feb. 5, 1946 2,410,429 Daman Nov. 5, 1946 FOREIGN PATENTS Number Country Date 639,773 Germany Dec. 12, 1936 

